Electrical storage tube



Dec- 9, 1958 R. w. SMITH 2,864,031

ELECTRICAL STORAGE TUBE Filed nec. zo, 195o ORNEY 2,864,031 ELECTRICAL STRAGE TUBE Roland Wright Smith, Princeton, N. l., assignor to Radio Corporation of America, a corporation of Delaware Application December 30, 1950, Serial N o. 203,7 05 5 Claims. (Cl. 315-12) This invention relates to improvements in electrical storage tubes of the type wherein information is laid down by the action of a cathode ray beam upon one side of a twosided storage electrode and is taken off by the action of another cathode ray beam upon the other side of the same electrode. ln particular, it is concerned with an improved storage electrode for use in such tubes.

L. Pensak in co-pending U. S. application 29,746, led May 28, 1948,. now abandoned, disclosed an electrical storage tube wherein two independent cathode ray beams bombard separate sides of a two-sided target electrode. This target, or storage electrode, comprises a conductive signal plate covered by a layer of insulating material which becomes electrically conductive in the area of impact when struck by electrons havingr suicient velocity. In the operation of the tube, a low velocity electron beam of the order of 1000 volts bombards the surface of the insulating material and causes secondary emission of electrons therefrom. This loss of electrons results in static charges between the surface of the insulating material and the conductive signal plate. A high velocity electron beam (about 16,000 volts) which is the writing beam penetrates into the insulating material making it conductive and discharging the potential between the insulating material surface and the conductive plate. When the low velocity or reading beam again charges up the surface of the insulating material, there is a signal current through an output circuit connected to the conductive plate. It has been found, however, that repeated high velocity electron bombardment and penetration of this insulating material causes changes in its physical characteristics which show up as undesirable signals or noise in the tube output. This is particularly noticeable when a steady signal (e. g. range markers) has been stored in a given area. When this particular signal is no longer desired, and after it is no longer included in the signal input to the tube, there still remains a certain amount of spurious signal output resulting from the effect of the previous steady bombardment of the storage electrode.

Accordingly, it is an object of the present invention to provide a storage tube having a longer life and more stable characteristics.

Another object is to provide an improved storage electrode and one characterized by its ability to resist deterioration when subjected to prolonged bombardment by an electron beam.

Another object is to provide a noise-free storage tube, and one which shall utilize lower operating voltages than storage tubes of the same general dimensions.

The improved storage electrode of the present invention, in a preferred embodiment, comprises a supporting layer of light-transparent material co-ated on one side with an electron-sensitive, light-emissive, phosphor and on the other side with a light-transparent conductive coating and a layer of photo-conductive material.

Bombardment of the surface of the photo-conductor by a first electron beam results in secondary emission from the conductor and charges of potential difference between the surface bombarded and the conductive coating. When the phosphor is excited by a second electron beam, it emits light and this light, by rendering the photo-conductor conductive, discharges the potential stored between the conductive coating and the surface of the photo-conductor in the immediate area of excitation,

Restoration of the charge by the first electron beam causes a current in the circuitry associated with the con ductive coating and photoconductor, and provides an output signal.

The invention is explained in more detail by reference to the drawing in which:

Fig. l is a longitudinal section of an electrical storage tube embodying the invention;

Fig. 2 is an enlarged view in section of the storage electrode of Fig. 1.

The tube of Fig. 1 comprises a substantially cylindrical glass envelope 11 containing a continuous evacuated space divided into a writing chamber 13 and a reading chamber 15 by a two-sided storage electrode or target 17 which is situated at the approximate center of the cylindrical envelope 11 and normal to its longitudinal axis.

At the end of the writing chamber 13 remote from target 17 is a high velocity electron gun 19 comprising a cathode 21, a control electrode 23, a screen grid 25 and a focusing anode 27. The interior surface of the glass envelope 11 in the region between the focusing anode 27 and the storage electrode 17 is coated with an electrically conductive substance 29 such as aquadag and serves as a second focusing and accelerating anode. The gun 19 is positioned axially of the chamber 13 so that its electron beam 31 (referred to as the writing beam 31) will follow apath transverse to the storage electrode 17. On the outer surface of the chamber 13 and in the region where the electron beam 31 emerges from the focusing anode 27 or deilecting coils 33 which cause the beam 31 to scan an image raster upon the storage electrode or target 17.

At the end of the reading chamber 15, remote from the storage electrode 17, is an electron gun 35 comprising a cathode 37, a control electrode 39 and an accelerating and focusing electrode 41. The interior surface of the reading chamber 15 in the region between the accelerating anode 41 and the storage electrode 17 is coated with an electrically conductive substance 42, such as aquadag, similar to the coating 29 in the writing chamber 13. The coating 42 is maintained at the same potential as the anode 41, and does not accelerate the electrons from the gun 35, but serves as a collector for secondary-electrons from the reading surface of the target 17. The gun 35 is positioned along the longitudinal axis of the chamber 15 so that its electron beam 43 (referred to as the reading beam 43) impinges upon the reading surface of the storage electrode 17. Deilecting coils 44 cause the electron beam 43 to scan an image raster upon said surface.

As shown more clearly in Fig. 2, the storage electrode 17 comprises a foundation plate 45 constituted of glass or equivalent transparent or translucent material (e. g. mica). The writing side of this transparent plate 45 supports an electron-sensitive, light-emitting, phosphor material 47 covered by an aluminum film 49. The other or reading surface of the transparent plate 45 is coated with a thin, semi-transparent conductive coating 51 and a layer of photo-conductive material 53.

In the operation of the tube, the reading beam 43 from the gun 35 in the reading chamber 15 bombards the surface 55 of the photo-conductive layer 53. Secondary electrons are emitted from the surface 55 and are collected upon the conductive coating 42 until surface 55 and coating 42 are brought to approximately the same potential This equalizing of potential takes place because the negative electrons which are freed from the surface 55 are attracted to the more positive coating 42 until the surface 55 loses so many electrons that it reaches the same positive potential as coating 42. This produces an electrostatic charge or potential difference between the-surface 55 and the thin conductive coating 51. The

d? surface 55 and the conductive coating 51 act like the charged plates of a condenser with the main body of the photo-conductive material 53 insulating them from each other.

Electrical `currents flowing through the deflection coils 33 cause the `electron beam from the writing gun i9 to scan an image raster upon the Writing surface of the storage electrode 17. This beam is of a high velocity and penetrates through the aluminum iilm 49 to excite the phosphor material 47. The phosphor in the area of electron impact emits light in accordance with signal input voltages applied to the control grid 23 of the writing gun 19. vThe light emanating from the phosphor l47 passes through the transparent foundation plate .45 and through the -transparent conductive coating 51 thereon. The light reaching the photo-conductive layer renders said layer conductive in the immediate area of excitation. As a consequence, a conductive path is established in kparticular areas for discharging the electrostatic charges between the conductive coating 51 and the surface 55 of the photo-conductive layer S3. The discharged areas of the surface 55 remain in that conditionuntil recharged by the reading beam 43. Because of the vinsulating qualities of the photo-conductor 53 the discharged areas of the surface 55 are not affected by the charge present between the conductive layer 53 and other areas of the surface 55 of the photo-conductor 53.

When the reading beam 43 scans the previously discharged areas, it recharges the surface 55 of the insulating material 53 to the potential of the conductive coating 42, in the manner described above for the original charging. This charging of the surface 55 to a positive potential with respect to the conductive coating 51 causes an electron flow from the battery 59 through an output resistance 57 to the conductive coating-51. The voltage variations across the resistance 57 become a signal output through the condenser 58. Since there will be some current through the resistance 57 during the Writing cycle when the writing beam 31 excites the phosphor 47 and causes a discharge between the surface 55 through the photo-conductor 53 to the conductive layer Si, an appropriate frequency separation circuit (not shown) may be employed to make the output circuit sensitive to only 'the reading beam. This type of circuit is Well known in the storage tube art and is explained by L. Pensak in an article entitled The Graphichon-A Picture Storage Tube published in the RCA Review for March 1949, cf. p. 67 ff.

In this manner, an electrical signal imposed upon the storage or target electrode 17 through the agency of the writing beam 31 can be stored on the electrode 17 until it is desired to remove the signal by the action of the reading beam 43.

In the selection of materials for the phosphor layer 47 and the photo-conductive layer 53, care should be taken that the spectral emission of the phosphor corresponds to the spectral sensitivity of the photo-conductor. With these requirements in mind, it is suggested that the phosphor layer 47 consist of blue-emitting zinc sulfide and the photo-conductive layer 53 of evaporated zinc sultide.

The function .of the aluminum tilm is explained in an article by D. W. Epstein and L. Pensak in the RCA Review for March ,1946, page 5, entitled Improved Cathode Ray Tubes With Metal-Backed luminescent Screens. Among other things it results in reection of the light produced by the phosphor toward the photo-conductor with a consequent increase in efliciency.

The tube is shown in Fig. 1 with the writing beam 31 operating at 10,000 volts. This is the same voltage as employed in the Pensak storage tube referred to above. Such high voltage on the Writing beam is not necessary, however, With the improved storage electrode of the present invention. The writing beam does not require the high velocity necessary for penetration of the insulating layer as in the Pensak construction. Sufficient velocity to excite the phosphors is all that is necessary. Satisfactory results have been secured with a writing beam as low as 1,000 volts.

Likewise, since the electron beam need not penetrate into the target to make it conductive, the structure described eliminatcs spurious'signals or noise resulting from physical deterioration of the storage electrode due to continuous bombardment, and provides a storage tube with a longer life.

What is claimed is:

1. A cathode ray device comprising an evacuated envelope containing a storage electrode which comprises a phosphor layer, a photo-conductive layer, and an interposed light-transparent electrically conductive layer; a first source of electrons for exciting said phosphor material in accordance with an input signal to cause said phosphor material to emit light upon said photo-conductive material; and a second source of electrons for creating an electrical charge between a surface of said photoconductive material and said electrical conductor when said phosphor is not emitting light.

2. A storage tube comprising an evacuated envelope containing an electrode having a translucent base of insulating material, a layer of electron-sensitive light emitting phosphor on one side of said base, a transparent conducting iilm on the other side of said base and a layer of photo-conductive material having a secondary-electron emissive characteristic on said conducting tilm; a rst electron gun mounted within said envelope in a position to bombard said light-emitting phosphor and a second electron gun mounted within said envelope in a position to bombard said photo-conductive material.

3. A-n electrical storage device comprising an evacuated envelope containing a storage electrode, said electrode having a layer of light-transparent electrically conductive material, a photo-conductive layer on one surface of said material, and a layer of electron-sensitive light emissive phosphor closely spaced to and insulated from the surface of. said light transparent electrically conductive material opposite said photo-conductive layer, cathode ray means for establishing a potential difference between said electrically conductive layer and a surface of said photoconductive layer, cathode ray means for causing said phosphor to emit light and hence to render conductive said photo-conductive layer, and an electrical lead connected with said electrically conductive layer for deriving a signal output, therefrom.

4. The invention according to claim 3 wherein said phosphor and said photo-conductive layer have similar spectral characteristics.

5. A compo-site device comprising in combination a conducting layer, luminescent means having said conducting layer mounted on the side thereof presented to exciting radiation, a light transparent continuous conducting layer, said light transparent layer being mounted on the opposite side of said luminescent means and being connected to a source of electrical potential, and a photoconductive layer, all the aforesaid parts forming together a multi-layer screen.

References Cited in the tile of this patent UNTED STATES PATENTS 2,169,840 Lewis Aug. 15, 1939 2,190,200 Victoreen Feb. 13, 1940 2,198,479 Langmuir Apr. 23, 1940 2,258,294 Luhszynski et al. Oct. 7, 1941 2,293,899 Hanson Aug. 25, 1942 2,516,314 Goldsmith July 25, 1950 2,525,768 Bruns Oct. 17, 1950 2,527,652 Pierce Oct. 31, 1950 2,540,632 Rose Feb. 6, 1951 2,544,753 Graham Mar. 13, 1951 2,549,072 Epstein Apr. 17, 1951 2,555,091 Lubszynski May 29, 1951 2,603,757 Sheldon July 15, 1952 

